KolabMailSynchronizer: imap folder UIDVALIDITY change tracking
KolabMailSynchronizer: imap folder UIDVALIDITY change tracking

* added folder imap UIDVALIDITY recording
* added tracking of UIDVALIDITY changes
* reduced the need for change flag propagation
* clear folder changed bit once all uids within
  that folder have been updated

GNOME/evolution-kolab,GNOME/evolution-kolab